4.2.2(c)(i)-(ii) - Kingdom and domain classification systems
Classification systems are models for organising the diversity of life. In this lesson, you will learn the features used in the five-kingdom system, why newer molecular evidence changed high-level classification, and how the three-domain system compares with the kingdom system. The key idea is not that old evidence was useless; it is that better evidence can change scientific explanations.
Classification As A Model
A classification system groups organisms using shared features. The aim is to make biological diversity easier to study, identify and compare. Earlier systems relied heavily on similarities that could be observed directly, such as body structure, cell structure, nutrition and whether an organism had a cell wall.
Classification System
A classification system is a way of arranging organisms into groups using shared features and evidence about their relationships.
The five-kingdom system was a major improvement on simply dividing living things into plants and animals. It separated organisms with very different cell organisation and nutrition, including fungi, protoctists and prokaryotes.
Observable features still matter. A student can often decide whether an unfamiliar organism is likely to be a plant, animal or fungus by looking at cell type, organisation and nutrition. The limitation is that appearance does not always reveal deep evolutionary relationships, especially among microorganisms that are small and outwardly simple.
The newer domain system shows a How Science Works point: scientific knowledge develops over time when new methods provide better evidence. Classification changed because microscopy, biochemistry and molecular biology gave scientists more precise evidence than appearance alone.
Five-Kingdom Features
The five kingdoms named in this specification are Prokaryotae, Protoctista, Fungi, Plantae and Animalia. The features used to classify organisms into these kingdoms are mainly observable or cellular features.
| Kingdom | Typical cell organisation | Cell wall | Nutrition and other useful features |
|---|---|---|---|
| Prokaryotae | prokaryotic; usually unicellular; no nucleus or membrane-bound organelles | usually present; bacterial cell walls contain peptidoglycan | very varied metabolism; DNA is not enclosed in a nucleus |
| Protoctista | eukaryotic; mostly unicellular, though some are multicellular or colonial | variable; some have cell walls, some do not | may be photosynthetic or heterotrophic; organisms that do not fit neatly into plants, animals or fungi |
| Fungi | eukaryotic; unicellular or multicellular, often as hyphae | chitin | heterotrophic; often feed by extracellular digestion and absorption |
| Plantae | eukaryotic; multicellular | cellulose | photosynthetic; contain chloroplasts and chlorophyll |
| Animalia | eukaryotic; multicellular | absent | heterotrophic; usually ingest food and have specialised tissues |
For kingdom classification, avoid using one feature in isolation when the stem gives several clues. For example, a cell wall alone does not prove "plant", because fungi and many protoctists also have cell walls.
The difference between prokaryotic and eukaryotic cells is especially important. Prokaryotic cells do not have a nucleus or membrane-bound organelles. Eukaryotic cells do have a nucleus and membrane-bound organelles, so Protoctista, Fungi, Plantae and Animalia are all eukaryotic kingdoms.
The kingdom Protoctista is broad. It contains eukaryotic organisms that are not animals, plants or fungi. That makes it useful for simple classification, but it also hints at a problem: a broad "does not fit elsewhere" group may not show exact relationships very well.
Molecular Evidence And New Systems
Observable features were the original evidence for classification. More recent classification uses similarities in biological molecules and genetic evidence. This can include DNA base sequences, RNA sequences and amino acid sequences in proteins.
Molecular evidence is powerful because molecules can be compared across very different organisms. Ribosomal RNA is especially useful for deep relationships because all cellular organisms have ribosomes, and the RNA in ribosomes changes slowly enough to compare distantly related groups.
Using molecular evidence
Suppose two microorganisms both look like simple prokaryotic cells under a microscope. If their ribosomal RNA sequences are very different, appearance alone is not enough evidence to place them in one close group.
A stronger conclusion would be: both organisms are prokaryotic in cell organisation, but molecular evidence suggests they belong to different high-level groups.
This is the key reason the old Prokaryotae grouping was revised. Bacteria and archaea are both prokaryotic in cell organisation, so they look more similar to each other than to animals or plants. However, molecular evidence showed that archaea are not simply another type of bacterium. They have important differences in ribosomal RNA, cell-wall chemistry and other molecular features.
Observable features are useful for broad sorting. Molecular and genetic evidence can reveal relationships that observable features alone can hide.
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